Bolzon B

Bravin E

Burger S

Dabrowski A

Lefèvre T

Olvegård M

Welsch CP

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Cockcroft-11-09      PERFORMANCE OF PARABOLIC AND DIFFUSIVE OTR SCREENS AT THE CLIC TEST FACILITY 3    M. Olvegård, B. Bolzon, E. Bravin, S. Burger, A. Dabrowski, and T. Lefèvre CERN, Geneva, Switzerland, C.P. Welsch, University of Liverpool, United Kingdom PERFORMANCE OF PARABOLIC AND DIFFUSIVE OTR SCREENSAT THE CLIC TEST FACILITY 3M. Olvegård∗, B. Bolzon, E. Bravin, S. Burger, A. Dabrowski, and T. LefèvreCERN, Geneva, Switzerland,C.P. Welsch, University of Liverpool, United KingdomAbstractAt the CLIC Test Facility 3, OTR screens are commonlyused in beam imaging systems for energy and energyspread characterization in dedicated spectrometer lines. Inthese lines the horizontal beam size is typically of the or-der of one centimeter. Already in 2005 a limitation wasobserved resulting from a strong dependence of the inten-sity of the light captured by the camera, on the positionon the screen (vignetting). The severity of this effect in-creases with the electron energy, as the aperture of the op-tical system is finite and the OTR photons are emitted in asmall cone of 1/γ angle. To mitigate this effect, differentshapes and surface polishing of the screens were investi-gated. Parabolic and diffusive OTR radiators were testedin several spectrometer lines all along the CTF3 complex.The results are presented in this paper.INTRODUCTIONOptical transition radiation (OTR) has become a popularmethod of beam imaging, ever since it was first introducedin beam diagnostics applications in the late 70s by Wartski[1]. OTR is emitted when a charged particle meets a bound-ary between two materials of different dielectric properties.If a conducting foil intercepts a particle’s path, a cone oflight is emitted, around the specular direction, as the par-ticle enters the foil (backward OTR) and another when itexits the foil (forward OTR), see Figure 1.−0.1 −0.05 0 0.05 0.10246θ (rad)intensity (arb.u.)  20 MeV65 MeV150 MeVFigure 1: OTR emission at the entrance and exit of a screen,and the corresponding angular distribution around the spec-ular directionThough, the light yield is generally low, in comparisonto e.g. scintillating light, it has other strong advantages forbeam imaging, such as perfect linearity to the number ofcharge crossings (i.e. no risk of saturation) and the possi-bility of a femtosecond time resolution.The angular distribution of the OTR intensity, for ultra-relativistic particles, is given by the expression in Equa-∗Supported by the EU under contract PITN-GA-2008-215080.tion 1 [1]. The peak intensity occurs at θmax = 1/γ, a peakwhich is more pronounced for higher particle energies.d2IdωdΩ=q2π2cθ2(γ−2 + θ2)2(1)Figure 1 shows the OTR intensity distribution for an elec-tron at 20MeV, 65MeV and 150MeV, which are typicalbeam energies at the different spectrometer lines at CTF3.SPECTROMETER SCREENS AT CTF3A standard OTR imaging system in the CTF3 [2]spectrometer lines consists of an aluminum screen150mm×50mm large, 50−200μm thick, intercepting thebeam path at a 45◦ angle. The backward OTR light isguided to a CCD camera, through an optical line of mirrors,achromatic lenses, and filters for light attenuation. The typ-ical spatial resolution of such system is a few hundred mi-crons, normally determined by the optical magnification ofthe system and of the image digitizer [3].VignettingThe optical aperture of the first lens is generally the lim-iting factor for the overall acceptance of the system. Unlikee.g. scintillating light, which can be seen as an isotropiclight source, OTR is highly directional, as was illustratedby the distribution in Figure 1. This has an undesired con-sequence that becomes worse with increasing beam energy,illustrated in Figure 2. For low energy particles the light isso divergent that, irrespective of the emission point, someof the light will fall within the optical aperture. For highenergy particles, however, the angle of maximum emis-sion becomes smaller, and most of the OTR will be emitedwithin a small cone, given by Equation 1. Therefore, lightgenerated at the screen center may be completely detected,while light generated further away from the center may es-cape the image system partially or entirely.In optics, the term vignetting is used to describe the sit-uation where less light is collected from the edges of anoptical system. Similarly in this case the amount of lightreaching the imaging system (CCD) will depend on whereit was generated, which means that large beams at higherenergy, as in the case of most of the CTF3 spectrometerscreens, will not be imaged properly.The relative illumination of the CCD camera as a func-tion of emission point at the screen, has been simulated inZemax [4] for typical CTF3 beam energies. The result leadto a study of how to best reduced this effect [5].Proceedings of DIPAC2011, Hamburg, Germany TUPD4905 Transverse Profiles 1Figure 2: The higher the beam energy, the more pro-nounced light loss from the edges of the system.MITIGATION METHODSTo some extent, OTR emission can be seen as a two-stepprocess: i) generation and ii) reflection from the surface,where i) is determined by the dielectric properties of theradiator, and ii) is given by the surface reflectivity. Two dif-ferent methods to mitigate the vignetting effect were triedat CTF3, attacking ii) from opposite directions: concen-trating the light onto the optical aperture, or an intentionaldiffusion of the light at the point of emission.Parabolic ScreensThe idea of using a parabolic screen support is to not letany light escape the first lens in the optical line between theradiator and the detector. By choosing a curvature givenby y = x2/(4f), with f being the approximate distancebetween radiator and the first lens, the emitted light is fo-cused onto the lens. No matter where on the screen the lightemanates from, it will have a great chance of reaching thecamera. Note that the curvature is modest enough to avoidany significant distortion of the image.Parabolic screen supports, covered by a 50−100μm thinaluminum foil were installed at three locations at CTF3:one in the CTF3 Linac and two in the CLIC Experimentalarea (CLEX), see Table 1.Diffusive ScreenBy increasing the diffusivity of the radiator surface theaverage angular distribution of the total reflected light isincreased. This means that the emission becomes almostisotropic and the vignetting effect is reduced. However,this implies an overall light loss, which is why it requiresgood margins in terms of initial light yield. The high chargebeam at CTF3 offers ideal conditions for such measures.An extensive test of how to best produce screens witha controlled diffusivity was performed in 2007 and is de-scribed in [5]. Polished, reflecting surfaces were then in-tentionally de-polished by various methods, until the reflec-tivity of the material was significantly reduced. The resultof these tests, led to the installation of diffusive aluminumscreens at two locations at CTF3: one after the Delay Loop(DL) and one in CLEX, see Table 1.MEASUREMENT METHODIn order to quantify the improvement of the new screens,systematic measurements on four screen systems at CTF3Table 1: OTR Systems for Spectrometry at CTF3Name Location Energy TypeCLS 0440 Linac (20MeV) flatCLS 1050 Linac 72MeV parabolicCTS 0840 DL 112MeV diffusiveCCS 0980 CLEX 112MeV parabolicCBS 0300 CLEX 120MeV diffusiveCMS 0630 CLEX (150MeV) parabolicCTS 0455 (removed) 105MeV flathave been made. The performance of each screen has beeninvestigated using a dipole scan technique: The beam ismoved across the screen by changing the dipole magnetcurrent in small steps and, for each current setting, an im-age is acquired. Assuming that the beam properties other-wise stay constant, these images can reveal how well thescreen reflects the beam profile depending on position.An example of how the quality of the measurement canvary from one system to the next can be seen in Figure 3.It shows a few horizontal projections from dipole scans on(a) a standard flat screen, formerly in use at CTF3, and b)a diffusive screen. Both the amplitude and the shape of theprofile may vary. In order to minimize sensitivity to noise,a Gaussian fit to each individual projection has been made,and by extracting the position, amplitude, and width of theGaussian, the variation can be quantified.−20 −10 0 10 200255075100x (mm)intensity (%)CTS 0455flat screena)−40 −20 0 20 400255075100x (mm)intensity (%)CBS 0300 b)Figure 3: The horizontal projection of screen images fordifferent dipole current settings with a standard flat screen(left) and a diffusive screen (right)RESULTSResults from the measurements are displayed in Fig-ure 4. Plots a) and b) show the relative, total, light intensity,detected by the CCD, for different peak positions x. Thefinal effect that this has on the measured beam momentumspread is demonstrated in plot c) and d). Here, the rela-tive deviation from a reference momentum spread is plot-ted, with a well centered measurement, or, for CCS 0980,the position that gives the maximum intensity, used as a ref-erence. The momentum and momentum spread have beencalculated from the magnetic field strength and the disper-sion at the position of the screen. For comparison, the plotsat the top also contain the corresponding data from a flat,polished screen (CTS 0455 in Table 1).TUPD49 Proceedings of DIPAC2011, Hamburg, Germany2 05 Transverse Profiles−40 −20 0 20 400255074100x (mm)intensity (%)  a)CCS 0980CLS 1050flat screen−40 −20 0 20 400255074100x (mm)intensity (%)  b)CBS 0300CTS 0840flat screen−40 −20 0 20 40−20020x (mm)(Δp−Δp0)/Δp 0 (%)  c)CCS 0980CLS 1050−40 −20 0 20 40−20020x (mm)(Δp−Δp0)/Δp 0 (%)  d)CBS 0300CTS 0840Figure 4: Results from a dipole scan measurement on fourscreens: two parabolic, plots a) and c); and two diffusive,plots b) and d), with a flat, polished screen. for comparison.Parabolic ScreensThe parabolic screens demonstrate a clear position de-pendence on the relative integrated intensity seen by thecamera, plotted in Figure 4 a). Not only is the relative in-tensity as low as 30% at a certain position, but there alsoseems to be a misalignment of both the parabolic screens.One of the screens, CCS 0980, has its maximum response30mm away from the center, and, with the beam centered,an intensity level which is hardly distinguishable. Note,though, that the light loss on the edges is less abrupt thanfor the standard flat screen.Also, the energy spread obtained with the parabolicscreen CCS 0980 is position dependent, which, once again,indicates that the alignment of the system needs to belooked into. CLS 1050 exhibit a momentum spread mea-surement good within 10%, in Figure 4 c). Part of thisdeviation, but not all, comes from difficulties in making agood fit due to high noise levels.Diffusive ScreensBoth of the diffusive screens in Figure 4 b) give muchmore promising results: an intensity variation of < 10% atthe very center of the screens, and < 20% over a horizontalrange of ±20mm. The improvement from the standard flatscreen is remarkable.The best result is offered by CBS 0300, for which thedeviation in energy spread is less than ±6% from the refer-ence, in Figure 4 d). The spread measured with the seconddiffusive screen, CTS 0840, varies with up to 20%. Theorigin of the discrepancy in performance may be due to adifference in the optical line or, possibly, a misalignment.Further investigations, including Zemax simulations of theoptical lines, will be needed for a full understanding of thedifferences between the systems.If a vignetting problem is present, a beam well centeredon the screen should appear smaller than it actually is. An-other method of characterizing the system performance istherefore a comparison between a single-shot measurement−60 −40 −20 0 20 40 6000.51x (mm)intensity (%)CLS 1050  a) single−shotdipole scan−20 −10 0 10 2000.51x (mm)intensity (%)CBS 0300  b) single−shotdipole scanFigure 5: A good agreement between dipole scan andsingle-shot measurement, both for a parabolic (left) and adiffusive (right) screen.and a measurement where only a slice of the screen is usedfor scanning the beam. The intensity on the slice is plottedas a function of equivalent screen position, correspondingto a given dipole current, which is extracted from how theGaussian peak changes with the dipole current.Figure 5 b) shows an example of such a comparison donefor CBS 0300. The profile obtained from a dipole scan isaccurately reproduced with a single-shot measurement. Asimilar comparison with CTS 0840 and the parabolic CLS1050, Figure 5 a), give also a good agreement, while thecomparison could not be made for the parabolic CCS 0980,due to problems already noted above.CONCLUSIONOTR screens are indispensable tools in the daily opera-tion of CTF3, in particular for energy spread measurementsthrough spectrometry. Studies of how screen shapes andsurface conditions can improve the overall performance ofthe OTR based diagnostics systems have been performedin connection to CTF3. It has been found that non-linearresponse of spectrometer screens due to a vignetting effectin lenses can be reduced either by mounting screen foils onparabolic support (for initial focusing of the emitted light)or by making the radiator surface diffusive (intentionalincrease of light divergence). Measurements show thatparabolic screens, although less subject to the vignettingeffect previously observed with polished flat screens, aresensitive to misalignment. Diffusive screens, on the otherhand, offer excellent mitigation of the vignetting effect.Considering the improvement in performance that diffusiveradiators constitute, and how easily they can be manufac-tured and installed, these should be the primary choice forspectrometer screens, where the beam intensity allows it.REFERENCES[1] L. Wartski, Thèse de Doctorat, Universite de Paris-Sud, Or-say (1976)[2] G. Geschonke and A. Ghigo Eds, “CTF3 Design Report”,CERN/PS 2002-008 (RF).[3] T. Lefèvre et. al. “Time Resolved Spectrometry on the CLICTest Facility 3”, TUPCH083, EPAC’06, Edinburgh, June 2006[4] ZEMAX software, http://www.zemax.com[5] C.P. Welsch et. al. “Investigations of OTR Screen Surfacesand Shapes”, TUPCH089, EPAC’06, Edinburgh, June 2006Proceedings of DIPAC2011, Hamburg, Germany TUPD4905 Transverse Profiles 3

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Performance of Parabolic and Diffusive OTR Screens at the Clic Test Facility 3

Performance of Parabolic and Diffusive OTR Screens at the Clic Test Facility 3

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